1. Field of the Invention
The present invention relates to mixing and dispensing devices, and, more particularly, to plastic raw materials mixers for use in conjunction with plastic materials processing machines, such as extruders and injection molding machines, where a plurality of ingredients of granular and/or pulverulent consistency are combined at predetermined ratios and fed to the processing machine.
2. Description of the Prior Art
Different methods may be used to automatically supply a predetermined quantity of a given mixture of plastic raw materials to a plastics processing machine. One approach involves the separately metered supply of each ingredient to a mixing receptacle, at a rate which is controlled by the rate at which the mixture is consumed. This approach requires complex and costly metering equipment, if ready adaptability to different production rates and changing raw material compositions is desired.
Another approach, featured by the present invention, involves the self-interrupting gravity feed of the major raw material component into the mixer receptacle in combination with the metered, machine-paced supply of one or more additives, such as pigments and the like, to the mixer. In order to achieve such a self-interrupting gravity feed, this type of mixing device commonly has a dip tube which reaches into the mixer receptacle from above, and through which the raw material enters the receptacle by gravity, the flow being blocked, or at least slowed, when the level of the materials inside the mixer receptacle reaches the opening of the dip tube.
One such prior art device is disclosed in U.S. Pat. No. 830,543, the device featuring three dip tubes discharging raw materials into separate regions of the mixer receptacle. As the various raw materials flow downwardly through the dip tubes into the mixer receptacle, the latter is rotated around a vertical axis, so that the raw materials are deposited in the receptacle along horizontal circular paths. The gravity discharge of raw materials into the mixer receptacle continues until the level of the materials inside the receptacle reaches the dip tube openings, thereby first slowing and eventually stopping further discharge into the receptacle, when the dip tube openings are completely covered by previously discharged materials. As a portion of the mixed materials is consumed by the plastics processing machine, the materials level inside the mixer receptacle falls, thereby allowing proportionate amounts of raw materials to be discharged from the dip tubes.
While this type of arrangement works satisfactorily with certain granular materials, it has been found to malfunction with other materials, particularly with very freely flowing ingredients which have a tendency to continue flowing into the mixer receptacle, even though the materials level inside the latter is covering the opening of the dip tube. The result is an undesirably high fill level inside the mixer receptacle, accompanied by a distortion of the mixing ratios.
Among past attempts at a solution to this problem is the suggestion of orienting the discharge opening of the dip tube against the path of the materials which rotate in the mixer receptacle by either arranging an oblique opening on the dip tube, or by using an elbow-type dip tube. In practical use, however, both of these suggestions have fallen short of expectations, because they still require adjustment, in order to compensate for changes in the speed of rotation of the mixer and for differences in the flowability of the various raw materials which are to be processed. These adjustments are tricky and difficult to obtain with consistency.